How do I add plus when I have already filed and have been refunded?

Was the 2018 return prepared with TurboTax?   We can tell you how to upgrade an already-filed return to PLUS and pay the 19.99 by credit/debit card.

Why are you wanting PLUS?  Are you trying to unlock prior returns?  If so, have you located the prior returns, and are they in the SAME account that you used for your 2018 return?

We can tell you what to do when you answer all the above.

Be aware there's a free option if a tax return transcript would suit your purpose.   You may be able to immediately download a prior-year return transcript at the IRS website at the link below.  If you don't qualify to open a free IRS account there, you can order one to come by postal mail, which normally takes 5-10 calendar days.   
<a rel="nofollow" target="_blank" href="http://www.irs.gov/Individuals/Get-Transcript">http://www.irs.gov/Individuals/Get-Transcript</a>

The return was with Turbo Tax. I have read through what seems like hundreds of these questions, but what I am asking is how to add plus to access prior years if my latest tax return was accepted and a refund has been issued and I have received it. Everything is in the same account, I just have not had a need for Plus until now.

I know exactly what you are doing, as I said above.  But in order to give the proper steps, I first had to make sure the 2018 return was filed with TurboTax, and I had to make sure it was in the same account, because that is the only way it will work.   Watch for my answer with the complete steps.

Here's how to open an already-filed 2018 return in order to upgrade to PLUS.  You've already confirmed that the prior returns are in the same account.

• Log in and at the Tax Home go to the section "Your Tax Returns & Documents".   If necessary, click on that line or the word SHOW to expand that section.
• Choose 2018.
• Then click the link "Add a State".  (you aren't really adding one; that just opens the return back up.)
• Once the return is open, click in the left menu column on Upgrade.
• At the Upgrade screen choose PLUS.  NOTE:  If PLUS is grayed out, that means you are already in PLUS.  In that case, do NOT upgrade any farther, or you'll have problems.
• Once you are in PLUS, then you need to pay for it.
• In the left menu column, click on TAX TOOLS, then choose PRINT CENTER.
• In the Print Center choose "Print, save, view THIS YEAR's return."   (yes, you say "this year's" because you are paying for PLUS 2018 to unlock the prior returns.)  You don't really have to print it.
  
• On the next screen choose to PAY NOW.  (for this unlocking purpose, you cannot use the "Pay Later" feature if it's offered.)
• The next screen will be an Order Summary.  It should show PLUS for 19.99.
• Next screen says "Pay with Credit Card", but it can also be a debit card or prepaid debit card.

From your vote, I trust you got it, which is good news.  Good luck.